Some vehicles, such as semi-trucks, have a higher ride height than passenger cars creating potential for a front end of the passenger car to slide under a rear end of the taller vehicle during a collision. This is known as underriding. Accordingly, the United States government has enacted vehicle safety standards that require rear impact guards (also known as underride bumpers) on certain classes of vehicles to reduce instances of underriding. These vehicle safety standards can be found in the Federal Motor Vehicle Safety Standards (FMVSS) codified in Title 49 of the Codified Federal Rules. See FMVSS 223 and FMVSS 224 for specific requirements.
Typical rear impact guards include a horizontal bar rigidly connected to a frame of the vehicle by two or more vertical bars. The horizontal bar is located at height estimated to contact a front bumper of the passenger car during a collision to reduce the likelihood of underriding.